gdb: Move la_language into the language_defn class

gdb: Move la_language into the language_defn class

Move the language_data::la_language member variable into the
langage_defn class.

I have not made the la_language member variable a method of
langage_defn simply because of the large number of places that
la_language is referenced throughout GDB.  I have made the new member
variable constant though, so this should prevent accidental
assignment.

In the future we might consider converting la_language to a method,
but right now my goal is to remove the langage_data class, so I'm
happy to leave la_language as a constant member variable.

There should be no user visible changes after this commit.

gdb/ChangeLog:

	* ada-lang.c (ada_language_data): Remove la_language initializer.
	* c-lang.c (c_language_data): Likewise.
	(cplus_language_data): Likewise.
	(asm_language_data): Likewise.
	(minimal_language_data): Likewise.
	* d-lang.c (d_language_data): Likewise.
	* f-lang.c (f_language_data): Likewise.
	* go-lang.c (go_language_data): Likewise.
	* language.c (unknown_language_data): Likewise.
	(auto_language_data): Likewise.
	* language.h (language_data): Remove la_language field.
	(language_defn::language_defn): Initialise la_language field.
	(language_defn::la_language): New member variable.
	* m2-lang.c (m2_language_data): Remove la_language field.
	* objc-lang.c (objc_language_data): Likewise.
	* opencl-lang.c (opencl_language_data): Likewise.
	* p-lang.c (pascal_language_data): Likewise.
	* rust-lang.c (rust_language_data): Likewise.